The present invention relates to galvanic cells, and more particularly to improved means for venting sealed galvanic cells when inordinate pressure is generated therein.
A common type of sealed galvanic cell comprises a relatively deep, tubular envelope, usually drawn from metal, and a separate cover or end cap which is used to close the envelope after the latter is filled with electrolyte, separators, and other materials depending upon the nature of the cell. Both Leclanche and alkaline cells have been formed in this manner. Although such cells have been produced in a vast array of shapes and sizes, perhaps the most common type of cell constitutes the relatively small, cylindrical sealed cell often broadly termed a "flashlight battery". Of course, sealed cells of the type described are used in countless other types of apparatus, and are formed in many different sizes and capacities.
The construction of such cells is far from simple, and must meet exacting demands with respect to both insulating and sealing capability. In particular, the cell construction must not only be economical, but must be liquid-tight to prevent the escape of the cell contents.
Generally speaking a sealed galvanic cell will theoretically operate at a relatively low internal pressure, which seldom exceeds approximately 11/2 atmospheres (absolute). In use, however, this figure is often exceeded. One particularly common cause of severe over pressure is the inadvertent reversal of a cell in an application wherein it is series-connected with two or more other cells. In such a case, the potential of the correctly-aligned cells overcomes that of the reversed cell and causes the reversed cell to charge rapidly. The cell electrolyte then "outgases", gas being developed much more rapidly than it can be re-absorbed by the cell contents. As a result pressure rapidly builds up within the cell and, if not relieved, may cause the cell to burst, dispersing the cell electrolyte and possibly damaging the enclosure in which the cells are disposed.
For the foregoing reasons it has become commonplace to provide venting or pressure relief mechanisms in sealed cells. Such venting mechanisms ordinarily allow gas to escape from the cell under overpressure conditions along a predetermined path, and are intended to relieve gas pressure before the cell bursts. While in principle the provision of a pressure-relief vent is straightforward, due to the small size and relatively low price of the more popular-sized cells the design of appropriate venting mechanisms is difficult.
Basically, three types of venting systems have evolved for use in small sealed cells. With one approach a spring-loaded valve is provided, the valve being biased by the spring with a pressure which is overcome during outgassing conditions, temporarily opening the valve against the pressure of its spring. With another approach a venting passageway is covered by a resilient member, such as a plastic washer. The resilient member can be temporarily deformed by excess pressure within the cell, forcing it away from the venting opening and allowing it to escape from within the cell. Finally, a currently popular approach with small-sized sealed cells is to provide a weakened section or diaphragm which is ruptured by unduly high pressures within the cell. In one version of this approach, the diaphragm is simply stretched to the bursting point by internal pressure; in another, the diaphragm is urged outwardly against a pointed anvil or the like which punctures the diaphragm, allowing the pent-up gases to escape. The latter two approaches have found favor with manufacturers of small sealed cells due at least in part to their simplicity, which allows them to be easily miniaturized for use with small cells. However, application of these or other known prior art systems to the smallest cells, for instance AA size cells, has been extremely difficult. Cells of the AA size conventionally exhibit a diameter in the range of one half inch. The center electrode or "nail", in order to have the rigidity necessary to withstand various manufacturing operations, and to make adequate contact with an upper electrode member, conventionally has a diameter of approximately one sixteenth inch. When constraints provided by still other structural elements are considered, very little space is left for a vent. Still further, due to the extremely small size of the cell complex mechanisms are precluded, and diaphragms and the like are extremely difficult to execute due to the extreme precision which is required. Further, the very small surface area afforded by the axial ends of the cell dictates that diaphragms, if used, be of extremely small area which further increases the criticality of their dimensions and lessens the tolerances of the structure.
Accordingly, it will be seen that it would be highly desirable to provide an improved venting structure for small, sealed galvanic cells.
It is accordingly an object of the present invention to provide an improved venting system for sealed galvanic cells.
Another object of the invention is to provide a simple vent structure which can be economically adapted to operate at various strictly-defined ranges of pressure.
Yet another object is to provide a venting system that can be mass-manufactured at low cost.
Yet another object of the invention is to provide a venting system whose parameters can be easily changed to allow venting to occur over a broad range of pressures.